Port Roles And The Active Topology - Cisco 3020 - Catalyst Blade Switch Configuration Manual

Cisco catalyst blade switch 3020 for hp software configuration guide, rel. 12.2(25)sef1
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Chapter 14
Configuring MSTP
These sections describe how the RSTP works:
For configuration information, see the

Port Roles and the Active Topology

The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning the
active topology. The RSTP builds upon the IEEE 802.1D STP to select the switch with the highest switch
priority (lowest numerical priority value) as the root switch as described in the
and BPDUs" section on page
A port with the root or a designated port role is included in the active topology. A port with the alternate
or backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root
port and designated port immediately transition to the forwarding state while all alternate and backup
ports are always in the discarding state (equivalent to blocking in IEEE 802.1D). The port state controls
the operation of the forwarding and learning processes.
IEEE 802.1D and RSTP port states.
Table 14-2
Operational Status
Enabled
Enabled
Enabled
Enabled
Disabled
To be consistent with Cisco STP implementations, this guide defines the port state as blocking instead
of discarding. Designated ports start in the listening state.
OL-8915-01
Port Roles and the Active Topology, page 14-9
Rapid Convergence, page 14-10
Synchronization of Port Roles, page 14-11
Bridge Protocol Data Unit Format and Processing, page 14-12
13-3. Then the RSTP assigns one of these port roles to individual ports:
Root port—Provides the best path (lowest cost) when the switch forwards packets to the root switch.
Designated port—Connects to the designated switch, which incurs the lowest path cost when
forwarding packets from that LAN to the root switch. The port through which the designated switch
is attached to the LAN is called the designated port.
Alternate port—Offers an alternate path toward the root switch to that provided by the current root
port.
Backup port—Acts as a backup for the path provided by a designated port toward the leaves of the
spanning tree. A backup port can exist only when two ports are connected in a loopback by a
point-to-point link or when a switch has two or more connections to a shared LAN segment.
Disabled port—Has no role within the operation of the spanning tree.
Port State Comparison
STP Port State
(IEEE 802.1D)
Blocking
Listening
Learning
Forwarding
Disabled
"Configuring MSTP Features" section on page
Table 14-2
RSTP Port State
Discarding
Discarding
Learning
Forwarding
Discarding
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
Understanding RSTP
14-14.
"Spanning-Tree Topology
provides a comparison of
Is Port Included in the
Active Topology?
No
No
Yes
Yes
No
14-9

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